U.S. patent number 7,948,341 [Application Number 11/051,629] was granted by the patent office on 2011-05-24 for moisture protection for an electromagnetic coil.
This patent grant is currently assigned to Mettler-Toledo AG. Invention is credited to Hans-Rudolf Burkhard, Jean-Maurice Tellenbach, Volker Ziebart.
United States Patent |
7,948,341 |
Tellenbach , et al. |
May 24, 2011 |
Moisture protection for an electromagnetic coil
Abstract
A coil for an inductive sensor, such as a coil which is used in
a sensor that operates according to the principle of
electromagnetic force compensation for converting an amount of
force generated by a load applied to a force-measuring cell into an
electrical signal, is provided with protection against the
penetration of moisture. The protection includes a protective
covering with a surface-smoothing undercoating applied to the coil,
on which a second level of coverage is applied as a protective
coating against the penetration of moisture.
Inventors: |
Tellenbach; Jean-Maurice
(Hettlingen, CH), Ziebart; Volker (Schlatt,
CH), Burkhard; Hans-Rudolf (Wila, CH) |
Assignee: |
Mettler-Toledo AG (Greifensee,
CH)
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Family
ID: |
34639444 |
Appl.
No.: |
11/051,629 |
Filed: |
January 27, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050160830 A1 |
Jul 28, 2005 |
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Foreign Application Priority Data
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Jan 27, 2004 [EP] |
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04075197 |
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Current U.S.
Class: |
336/82; 336/90;
336/96; 336/92 |
Current CPC
Class: |
H01F
5/00 (20130101); G01G 7/02 (20130101); H01F
27/04 (20130101); H01F 27/02 (20130101) |
Current International
Class: |
H01F
27/02 (20060101) |
Field of
Search: |
;336/82,90,92,96 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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504 675 |
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Apr 1971 |
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CH |
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1 276 620 |
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Nov 1972 |
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GB |
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57089211 |
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Jun 1982 |
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JP |
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S57-89211 |
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Jun 1982 |
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JP |
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01068624 |
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Mar 1989 |
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JP |
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2518171 |
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Mar 1996 |
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JP |
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Primary Examiner: Mai; Anh T
Assistant Examiner: Baisa; Joselito
Attorney, Agent or Firm: Buchanan Ingersoll & Rooney
PC
Claims
The invention claimed is:
1. An electromagnetic coil comprising: a plurality of windings; and
a protective covering, the protective covering comprising: a first
coating comprising a surface-smoothing under coating disposed on
the windings; and a second multilayered coating disposed on the
first coating, the multilayered coating comprising at least one
barrier layer comprising an inorganic material and at least one
intermediate layer comprising an inorganic material, wherein the
second coating defines a barrier to the penetration of moisture of
a nature so as to prevent changes in sensitivity of the coil during
operation thereof caused by penetration of moisture, wherein the
barrier layers and intermediate layers alternate in sequence and
wherein the barrier layers and the intermediate layers comprise
different inorganic materials and/or different stoichiometric
compositions of an inorganic material comprising at least two
components and/or an inorganic material in which structural
parameters vary.
2. A coil according to claim 1, wherein the first coating has a
chemical composition that is different than both the at least one
barrier layer and the at least one intermediate layer.
3. An electromagnetic coil comprising: a plurality of windings; and
a protective covering, the protective covering comprising: a first
coating comprising a surface-smoothing under coating disposed on
the windings; and a second multilayered coating disposed on the
first coating, the multilayered coating comprising at least one
barrier layer and at least one intermediate layer, wherein the
second coating defines a barrier to the penetration of moisture of
a nature so as to prevent changes in sensitivity of the coil during
operation thereof caused by penetration of moisture, and wherein
the first coating has a chemical composition that is different than
both the at least one barrier layer and the at least one
intermediate layer.
4. Coil according to claim 3, wherein the windings comprise wire
having an insulating covering disposed thereon.
5. Coil according to claim 3, wherein the at least one barrier
layer is adjacent to the at least one intermediate layer, and the
at least one intermediate layers and the at least one barrier layer
have different chemical compositions.
6. Coil according to claim 3, wherein the barrier layers and
intermediate layers alternate in sequence.
7. Coil according to claim 6, wherein the barrier layers comprise
inorganic material and the intermediate layers comprise polymer
material.
8. Coil according to claim 6, wherein the barrier layers and the
intermediate layers comprise an inorganic material.
9. Coil according to claim 3, wherein the coil is arranged on a
coil former, and the coil former is at least partially covered by
the protective covering.
10. Coil according to claim 9, wherein the coil has an
encapsulation device and at least parts that are not sealed by
encapsulation, including connecting wires of the coil and/or if
applicable, boundary areas between a coil former and the
encapsulation device, are covered by the protective covering.
11. Coil according to claim 3, wherein the coil is used in a sensor
that is configured to operate according to a principle of
electromagnetic force compensation for converting an amount of
force generated by a load applied to a force-measuring cell into an
electrical signal (I.sub.cmp).
12. Force-measuring cell with a force-transmitting mechanism and
with a sensor that functions according to a principle of
electromagnetic force compensation for converting an amount of a
force generated by a weighing load into an electrical signal
(I.sub.cmp), wherein the sensor comprises: a coil that is movable
in one dimension in a magnetic field of a permanent magnet, the
coil being configured in accordance with claim 3.
13. Method of applying a protective covering to a coil of claim 3,
comprising: applying the first, surface-smoothing level of coverage
to the coil; and depositing on the first, surface-smoothing level
of coverage, the second level of coverage as a protective coating
against penetration of moisture.
14. Method according to claim 13, wherein applying of the
surface-smoothing level of coverage is performed by an immersion
process.
15. Method according to claim 13, wherein applying of the
protective coating is performed by a method of plasma-enhanced
chemical vapor deposition (PECVD).
16. Method according to claim 13, wherein applying of the
protective coating is performed by vapor deposition or sputtering,
wherein the coil is revolving during a coating process.
17. An electromagnetic coil comprising: a plurality of windings;
and a protective covering, the protective covering comprising: a
first coating comprising a surface-smoothing under coating disposed
on the windings; and a second coating disposed on the first
coating, the second coating comprises one or more material
characteristics varying throughout a the entire thickness of the
coating thereby defining a barrier to moisture penetration of a
nature so as to prevent changes in sensitivity of the coil during
operation thereof caused by penetration of moisture.
18. Coil according to claim 17, wherein the material
characteristics include chemical composition.
19. Coil according to claim 17, wherein the characteristics vary
continuously throughout the entire thickness of the coating.
20. Coil according to claim 17, wherein the windings comprise wire
having an insulating covering disposed thereon.
21. Coil according to claim 17, wherein the coil is used in a
sensor that is configured to operate according to a principle of
electromagnetic force compensation for converting an amount of
force generated by a load applied to a force-measuring cell into an
electrical signal (I.sub.cmp).
22. Force-measuring cell with a force-transmitting mechanism and
with a sensor that functions according to a principle of
electromagnetic force compensation for converting an amount of a
force generated by a weighing load into an electrical signal
(I.sub.cmp), wherein the sensor comprises: a coil that is movable
in one dimension in a magnetic field of a permanent magnet, the
coil being configured in accordance with claim 17.
23. An electromagnetic coil comprising: a plurality of windings;
and a protective covering, the protective covering comprising: a
first coating comprising a surface-smoothing under coating disposed
on the windings; and a second multilayered coating disposed on the
first coating, the multilayered coating comprising at least one
inorganic barrier layer and at least one intermediate polymer
layer, wherein the second coating defines a barrier to the
penetration of moisture of a nature so as to prevent changes in
sensitivity of the coil during operation thereof caused by
penetration of moisture and wherein the at least one intermediate
polymer layer seals micropores and hairline breaks in the least one
inorganic barrier layer.
24. Coil according to claim 23, wherein the multilayered coating
comprises at least three layers.
25. Coil according to claim 24, wherein the multilayered coating
comprises at least five layers.
26. A coil according to claim 23, wherein the first coating has a
chemical composition that is different than both the at least one
barrier layer and the at least one intermediate layer.
27. An electromagnetic coil comprising: a plurality of windings;
and a protective covering, the protective covering comprising: a
first coating comprising a surface-smoothing under coating disposed
on the windings; and a second coating disposed on the first
coating, the second coating deposited from sources having different
chemical compositions by increasing a rate of deposition from a
first source over time while decreasing a rate of deposition from
another source over time, thereby defining a gradient of chemical
composition varying continuously throughout the entire thickness of
the second coating.
28. A coli according to claim 27, wherein the second coating
defines a barrier to moisture penetration of a nature so as to
prevent changes in sensitivity of the coil during operation thereof
caused by penetration of moisture.
Description
RELATED APPLICATIONS
This application claims benefit to European Priority Patent
Application Serial Number 04 075 197.6, filed Jan. 27, 2004. This
priority application is hereby incorporated by reference in its
entirety.
BACKGROUND
1. Field
An electromagnetic coil for an inductive sensor is disclosed, such
as a coil of the type that is used in sensors that operate
according to the principle of electromagnetic force compensation,
wherein the coil is provided with protective means against the
penetration of moisture. A force-measuring cell with this type of
coil and a method of applying a protective coating to a coil are
also disclosed.
2. Background Information
An inductive sensor is based for example on the concept of a coil
that is movable in an inhomogeneous magnetic field, where a current
is induced in the coil when the latter moves in a direction of the
inhomogeneity. The induced current represents a measure for the
displacement of the coil and can be used to measure a force that is
causing the displacement.
In other inductive sensors of the type used with preference in
force-measuring cells, a current flows through a coil that is
movable in the homogeneous magnetic field of a permanent magnet. A
force acting on the coil causes a displacement of the coil which is
detected by an optical position sensor, whereupon a servo-control
circuit changes the flow of current in the coil to the magnitude
required to hold the coil in its original position again. The
change in the current is commensurate with the force that is to be
measured by the force-measuring cell. This measurement principle is
referred to as electromagnetic force compensation.
A force-measuring device that operates according to the foregoing
principle of electromagnetic force-compensation, which finds
application for example in the field of weighing technology,
includes a force-transfer device with a parallel-guiding mechanism
and in many cases a lever mechanism for the reduction of a force,
e.g. a load, to be transmitted. The sensor has a permanent magnet
with an air gap, with a coil being immersed in the magnetic field
in the air gap of the permanent magnet. If the force-measuring
device has a lever mechanism, the coil can be arranged at the
longer lever arm of the last lever. A coil of the type used here
has one or more windings of an insulated metal wire, normally a
copper wire. The electrical insulation of the wire is required in
order to avoid electrical contact between adjacent windings.
The coil is in many cases wound on a coil former which lends the
necessary degree of stability to the windings. However, there are
also so-called air coils whose shape is maintained by an adhesive
compound connecting the windings to each other.
Force-measuring cells that work according to the principle of
electromagnetic force compensation, in particular if they are
designed for a high measurement resolution, often have to meet the
requirement of a high sensitivity that does not change, for example
during the operation of a balance that is equipped with the
force-measuring cell.
Besides other parameters not specifically mentioned here, the coil
is also a contributing factor in a sensitivity change of the
force-measuring cell, which is due in particular to moisture
absorption of the insulating material that sheathes the coil wire.
Furthermore, moisture absorption of the wire insulation causes a
shift of the zero point of the force-measuring cell. The insulation
of the coil wire consists in almost all cases of a polymer, for
example of polyurethane, polyimide, or polyamidimide.
Moisture absorption of the insulating material of the coil wire, or
generally a change in the moisture content of the insulating
material which depends on the ambient humidity of the
force-measuring cell, can lead for example to leakage currents
between the windings. As another possible consequence of a moisture
absorption or moisture release, the insulating material can swell
up or shrink. The result is in both cases a geometric change of the
coil, particularly of the spacing between neighboring windings,
which will also lead to mechanical stress inside the coil. The
change in the moisture content is further accompanied by a weight
change of the coil.
All of these moisture-related effects can, overtime, cause changes
in the measuring result, if this type of coil is used in an
inductive measuring sensor. In particular in the case of a
force-measuring cell that operates according to the principle of
electromagnetic force compensation, the moisture absorption of the
insulating material of the coil wire is one of the factors that can
determine the sensitivity of the measuring cell.
The existing state of the art includes a variety of measures that
have been proposed or are used in practice to avoid or at least
reduce moisture absorption in a coil.
As an example, a ring-shaped sleeve of aluminum is shrink-fitted on
a coil that is wound on a coil former. Due to the thermal expansion
that occurs when the sleeve is heated up, it can be fitted over the
coil former on which the coil has been wound, and in the subsequent
cooling, the aluminum sleeve contracts itself snugly against the
coil and the coil former, whereby a certain sealing effect is
achieved against an exchange of moisture with the environment. In
some cases, if a higher degree of seal-tightness is required, the
sleeve is welded to the coil former, leaving only a passage opening
for the conductor leads to the coil, which requires additional
sealing by means of a lacquer.
In the effort to improve the measuring results in an environment
with relatively high humidity fluctuations, for example between 20%
and 80% relative air humidity, satisfactory results--at least in
force-measuring cells with relatively low sensitivity
requirements--are also achieved with a thick lacquer coating
applied directly to a coil and solidified by a thermal
treatment.
A watertight encapsulation of the coil component of the
electromagnetic unit of an electromagnetic balance is disclosed in
JP 25 18171 Y2, the disclosure of which is hereby incorporated by
reference in its entirety. The coil is arranged on a coil former
that is configured in such a way that the coil windings lie in a
recessed part of the coil former. The coil is enclosed inside the
recessed space of the coil former by means of a watertight ring
sleeve that is fitted to the coil former. A tightly holding snap
connection exists between the coil former and the watertight ring
sleeve.
Ring sleeves used as sealing enclosures have the drawback that the
overall weight of the coil is increased, but their most decisive
disadvantage is that the geometric dimension of the coil,
particularly its thickness, is strongly enlarged, so that the width
of the air gap of the permanent magnet has to be adapted
accordingly. However, enlarging the air gap of a magnet system that
is otherwise unchanged decreases the magnetic field in the air gap,
which has a negative effect on the sensitivity of the sensor. If
the same sensitivity is to be maintained with an enlarged air gap,
a larger magnet system will be required which will, however, be
more difficult to handle and more expensive to manufacture.
Furthermore, coils that are sealed in accordance with the existing
state of the art still require special sealing measures at the
passage opening for the conductor leads to the coil. The
aforementioned lacquer seal does not provide the necessary
seal-tightness to achieve the low moisture-absorption values that
are required for a high sensitivity in the case of force-measuring
cells with a high measurement resolution. The same can also be said
to the aforementioned method of sealing the entire coil with a
lacquer coating.
SUMMARY
A coil with a simple and highly effective protective shield against
moisture, is disclosed.
A coil for an inductive sensor, such as a coil of the type that is
used in sensors that operate according to the principle of
electromagnetic force compensation for converting an amount of
force generated by a load applied to a force-measuring cell into an
electrical signal, is provided with protective means against the
penetration of moisture. The protective means include a protective
covering with a first level of coverage being applied to the coil
to even out the surface of the coil, and a second level of coverage
arranged on top of the first level as a protective coating against
moisture penetration.
A first a first level of coverage is adhesively applied as
surface-smoothing base cover to the windings of the coil or to the
insulation thereof, respectively. The surface-smoothing base cover
fills up the gaps between the individual windings creating a
flattened surface with a much reduced surface structure compared to
the surface of the coil. The smoothened surface serves as base for
the actual moisture protection coating.
The protective coating can be adapted according to the demands on
the sensitivity of sensor including the coil. The protective
coating is quite thin compared to the surface-smoothing base cover,
as it is grown on top of a smoothened surface.
In an advantageous embodiment of a coil used in a force-measuring
cell with a force-measuring device and a sensor operating according
to the principle of electromagnetic force compensation for
converting the amount of force induced by a load into an electrical
signal, with the coil being movable in the magnetic field of a
permanent magnet in one dimension, the coil is provided with
protective measures against moisture penetration. These measures
include a protective coating with a first surface-smoothing base
cover applied directly to the coil and a second protective coating,
which acts as barrier against moisture penetration and is deposited
on top of the first surface-smoothing base cover.
In an advantageous embodiment, a protective coating of the type
just described is configured inhomogeneously over the range of the
coating thickness. This reduces the incidence of hairline breaks or
micro-pores in the protective coating.
In an exemplary embodiment, the protective coating is a
multilayered coating with an alternating sequence of layers with
strong barrier properties and intermediate layers. The intermediate
layers have the purpose to seal the remaining micro-pores and
hairline breaks or fissures in the respectively adjacent barrier
layers. As a result, micro-pores and hairline breaks will not
continue through the entire thickness of the coating but will be at
different locations in the next barrier layer, whereby a kind of
labyrinth is set up against the penetration of moisture.
As a particularly beneficial effect of a multilayered protective
coating that is underlaid with a surface-smoothing base cover,
strongly curved surfaces where an inorganic barrier layer would be
particularly prone to develop the aforementioned micro-pores or
hairline breaks because of localized stresses including in
particular thermal stresses, and where micro-pores and hairline
breaks would have a special tendency to attach themselves, are
rounded out on the one hand by the underlying base of the
protective coating, so that the occurrence of micro-pores and
micro-fractures is reduced from the outset, and on the other hand
the micro-pores and hairline breaks are covered by the intermediate
layers whereby the aforementioned labyrinth effect is set up
against the penetration of moisture.
In an advantageous embodiment, the protective coating has a
sequence of barrier layers alternating with intermediate polymer
layers. Intermediate polymer layers, in particular if they are
deposited from the liquid phase or from a liquid solution, have the
advantage that they further enhance the flattening effect of the
surface-smoothing undercoating, so that the subsequent barrier
layers are deposited on surfaces of progressively lower surface
roughness.
In a particularly advantageous embodiment, the protective coating
is made entirely of inorganic materials. A multilayered coating can
in this case be configured with a sequence of alternatingly
different inorganic materials and/or with a sequence of
alternatingly different stoichiometric compositions of an inorganic
material including, or consisting of, at least two components,
and/or with a sequence of alternatingly varying structural
parameters of an inorganic material.
It is a particularly favorable aspect of inorganic multilayered
protective coatings that they can be produced in a single work
operation and in a single coating-deposition apparatus,
respectively.
These inorganic protective coatings, in particular of individual
layers of the latter if the coating is configured as a multilayered
coating, have the special property that they have a tendency to
grow in a conforming manner on their underlying base surface when
they are applied by means of plasma-enhanced chemical vapor
deposition (PECVD). This means that the coating has a substantially
constant thickness independent of the angle at which different
locations of the underlying base surface are oriented in relation
to the orientation of the source during the deposition process.
Thus, even strongly curved surfaces, for example at the coil ends,
receive the same amount of coverage by the protective coating as
the side surfaces of the coil that are at least partially smoothed
out by the smoothing base cover. Accordingly, the protection effect
against moisture penetration is optimized.
In an exemplary embodiment, the protective coating can have a
sequence of silicon oxide layers and silicon nitride layers.
Protective coatings of this type are disclosed in WO 03/050894, the
disclosure of which is hereby incorporated by reference in its
entirety, as a means for covering electronic devices, in particular
indicating devices such as light-emitting devices or liquid crystal
displays.
In another exemplary embodiment, a protective coating especially of
an inorganic type is configured with a continuous variation of one
or more parameters, in particular of the chemical composition of
the coating material, over the range of the coating thickness.
This continuous variation can occur on the one hand in multilayered
coatings in such a way that the coating parameters are not changing
abruptly, but that their profile rather resembles a sine function.
On the other hand, the inhomogeneity can occur in a continuous
transition over the entire thickness of the protective coating, as
a gradient of one or more of the coating material parameters. This
variant is particularly preferred for producing thin protective
coatings.
In an example of a method for producing a protective coating with
at least one continuously varying parameter, the source changes its
material composition during the deposition process, or the
protective coating is deposited from two sources whose deposition
rates vary, i.e., the rate from one source increases as a function
of time while the rate from the other source decreases, and/or vice
versa.
As is self-evident, the coating thickness of a protective coating
is determined on the basis of the required barrier effect, which
has to be stronger in the case of sensors with a high measurement
sensitivity as opposed to sensors for applications with less
stringent requirements in regard to accuracy and/or sensitivity. In
the field of weighing technology, for example, an inductive sensor
with a high sensitivity is used with preference in a
force-measuring cell for officially certifiable balances or for a
comparator balance. The barrier effect is, however, also dependent
on the materials used for the protective coating and on how they
are arranged in the protective coating. In other words, the barrier
effect depends on the structure and configuration of the protective
coating. A protective coating should also have a sufficiently high
degree of elasticity so that it can conform to a slight dimensional
change in the coil due to thermal expansion without thereby losing
its barrier effect against the penetration of moisture. This can be
achieved particularly well through thin inorganic protective
coatings or barrier layers within a protective coating. Of course,
economic factors also come into play in determining the optimal
coating thickness of a protective coating and/or in determining the
number of layers to use in a protective coating.
The thickness of the second level of coverage of the covering,
which functions as protective coating, can be in the range from a
few hundred nanometers to a few microns, and the thickness of the
individual layers is of the order of 100 nanometers up to 500
nanometers. The thickness of the first, surface-smoothing base
cover is determined in accordance with the surface structure of the
coil which depends on the wire diameter. This undercoating can have
a thickness of a few microns ranging up towards a hundred
micron.
The coil in an inductive sensor as described above can be either an
air coil, which means that the windings are held together in coil
shape by an adhesive material, or it can be a coil with a coil
former on which the one or more coil wires are wound. In the former
case, the entire coil needs to be provided all around with a
protective coating, while in the latter case only the exposed parts
of the coil windings need to be coated, but the coating could also
be extended over the entire coil former.
A coil that is equipped with the protective coating can be
configured without an encapsulation of the type that belongs to the
existing state of the art as described hereinabove. Consequently, a
coil with a larger number of windings can be used with a given
width of the air gap in a permanent magnet, or a permanent magnet
with a narrower air gap can be selected for a sensor with a given
number of coil windings. Both measures can lead to a significant
improvement of the sensitivity of the inductive sensor.
In another embodiment, in a case where an encapsulation is desired,
the coil can be arranged on a coil former and provided with an
encapsulation device and at least the parts of the coil that are
not sealed by the encapsulation, such as the connecting wires
and/or the boundaries between the encapsulation and the coil former
can be covered by the protective coating.
In an exemplary embodiment, the protective coating can be provided
with a cover layer that shields the protective coating above all
from extraneous mechanical factors. The materials used for the
cover layer include, for example, polymers with an exceptionally
low moisture absorptivity, and they also include silicone.
A production method for a protective covering of a coil includes
the steps of applying a first level of coverage to the coil to
smooth out the surface, and to deposit on the surface-smoothing
base coating a second level of coverage functioning as a protective
coating against the penetration of moisture.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described hereinafter with reference to
examples that are schematically illustrated in the drawings,
wherein:
FIG. 1 illustrates a principle of an exemplary force-measuring
cell, shown in a side view, which functions according to the
principle of electromagnetic force compensation;
FIG. 2 illustrates the arrangement of an exemplary coil of the type
that is used in force-measuring cells according to FIG. 1, in FIG.
2a) with a state-of-the-art encapsulation in a perspective view; in
FIG. 2b) as a sectional view along the line I-I of FIG. 2a); in
FIG. 2c) as a detail view of the portion framed by the circle A in
FIG. 2b) with a protective coating in the area of the passage
opening to the electrical conductor leads; and in 2d) as a detail
view of the portion framed by the circle B in FIG. 2b);
FIGS. 3a-3c illustrate an exemplary coil arranged on a coil former
without encapsulation in a form of a representation analogous to
FIGS. 2a) through 2c);
FIGS. 4a-4c illustrate an exemplary air coil without encapsulation
in a form of a representation analogous to FIGS. 2a) through
2c);
FIG. 5 illustrates an exemplary multilayered coating arranged on
the outermost winding layer with a surface-smoothing undercoating,
shown in a strongly magnified sectional view; and
FIG. 6 illustrates an exemplary protective coating, featuring a
continuous variation of one or more material parameters, arranged
on the outermost winding layer with a surface-smoothing
undercoating, shown in a strongly magnified sectional view.
DETAILED DESCRIPTION
In an illustration of a pronounced schematic character, FIG. 1
shows an exemplary force-measuring cell 1 of a type that is
suitable for use in the field of weighing technology, where the
force-measuring cell 1 functions according to the principle of
electromagnetic force compensation. The force-measuring cell 1
includes a force-transmitting device with a parallel-guiding
mechanism that has a stationary portion 2 and a vertically
displaceable portion 3 that is movably constrained by a pair of
guide members 4 which are connected to the stationary portion 2 and
the vertically displaceable portion 3 by way of flexure joints 5.
The vertically displaceable portion 3 includes a cantilevered
extension 15 serving to receive a load that is to be measured. The
perpendicular vector component of the force that is caused by a
weighing load is transferred from the vertically displaceable
portion 3 to the short lever arm 8 of the lever 6 by way of a
coupling element 9. The lever 6 is supportively pivoted on a part
of the stationary portion 2 by means of a flexure fulcrum 7. The
force-measuring cell further includes a cup-shaped permanent magnet
system 10 which is mounted through a fixed connection on the
stationary portion 2 and which has an air gap 11. A coil 13
connected to the longer lever arm 12 of the lever 6 is arranged in
the air gap 11. A compensation current I.sub.cmp flows through the
coil 13, with the magnitude of the current I.sub.cmp depending on
the magnitude of the force acting on the lever 6. The position of
the lever 6 is measured by an electro-optical measuring device 14
that is connected to a servo device which regulates the
compensation current I.sub.cmp in response to the measurement
signals received so that the lever 6 is always held in the same
position, or is returned to the same position after it has been
displaced due to a change in the weighing load.
FIG. 2a) illustrates a coil arrangement 23 of the type that is used
in force-measuring cells 1 according to FIG. 1, with an
encapsulation by means of a sealing sleeve ring 17 in conformance
with the existing state of the art. The coil 13 is arranged on a
toroid-shaped coil former 16 and encapsulated moisture-tight by
means of the sealing sleeve ring 17 that ends flush with the coil
former 16. Only the opening 18 for the passage of the electrical
conductor leads 19 to the coil 13 is excepted from the
encapsulation and therefore forms an entry passage allowing
moisture to reach the coil and in particular the insulating
material of the coil, whereby the undesirable effects are caused
that were mentioned herein at the beginning, such as leakage
currents between neighboring coil windings or a change in the
geometry of the coil, in particular in the spacing between adjacent
windings due to the swelling of the insulating material.
In a sectional view along the line I-I of FIG. 2a), FIG. 2b)
illustrates how the individual windings 20 are arranged inside the
encapsulation that is constituted on one side by the coil former 16
and on the other side by the sealing sleeve ring 17. FIG. 2c)
illustrates the detail framed by the circle A in FIG. 2b), and FIG.
2d) illustrates the detail framed by the circle B in FIG. 2b). FIG.
2c) shows how the coil former 16 is covered at the top in the area
of the opening 18 by a protective covering with a surface-smoothing
undercoating 21 and a protective coating 22. The opening can be
additionally closed off with a lacquer, or the material of the
surface-smoothing coating is applied in the area of the opening in
such a way that the lacquer closes off the opening. If the entire
top side and underside of the coil former 16 are provided with the
protective covering, the boundary area 24 between the coil former
16 and the sealing sleeve ring 17 will likewise be sealed. In cases
where coil former 16 and the sealing sleeve ring 17 are joined only
through surface contact, sealing the boundary area 24 in this
manner has been found very useful as a means to reduce the
penetration of moisture.
FIG. 3a) gives a perspective view of a coil 13 without
encapsulation where the coil 13 is, however, arranged on a coil
former 16. A coil former 16 used in this arrangement consists
preferably of a non-magnetic material such as copper or aluminum.
FIG. 3b) represents a sectional view along the line II-II of the
coil 13 that is arranged on the coil former 16, and FIG. 3c)
represents a magnified view of the detail A of FIG. 3b). FIG. 3c)
explains how the protective covering with the surface-smoothing
undercoating 21 and the protective coating 22 is arranged on the
outside of the coil 13 as well as on at least a part of the topside
(and analogously, the underside) of the coil former 16. The coil
former 16 does not have to be coated in its entirety, given that it
includes, or consists of, a metal, for example copper, which is
known to be moisture-tight.
FIG. 4 illustrates an air-cored coil in an analogous form of
representation as FIG. 3. An air-cored coil or air coil in the
present context means a coil 13 that is not supported by a coil
former 16. The protective covering which, in this case too, is
composed of a surface-smoothing undercoating 21 and a protective
coating 22 needs to be deposited on all surfaces of the air coil
13.
The material in the protective coating 22 is not necessarily of a
homogenous composition, stoichiometry, or structure. As has been
shown, inhomogeneous protective coatings have a highly efficient
barrier effect against moisture penetration. Inhomogeneous coatings
that may be considered include multilayered coatings as well as
coatings with a continuous variation of one or more material
parameters, in particular of the chemical composition, over the
range of the coating thickness.
A multilayered coating can, includes, or consist of, individual
layers of materials with a strong barrier effect, such as inorganic
materials, that are arranged in the protective coating in an
alternating sequence with intermediate polymer layers. The
intermediate polymer layers serve to cover micro-pores and hairline
breaks of the adjacent barrier layers in order to achieve overall a
strong barrier effect of the protective coating.
FIG. 5 illustrates an example of a protective covering on a coil in
the form of a multilayered coating, showing a strongly magnified
detail of the coil surface with an outer layer of windings 20 of a
wire 28 with an insulation 29. The outer layer of windings is
provided with a protective covering that includes a surface
smoothing undercoating 21 and a multilayered coating 25. The
multilayered coating 25 in the illustrated example consists of four
individual layers. However, the number of layers in a multilayered
coating 25 is not determined as an a priori defined value but
depends on the requirements of a maximally tolerable moisture
permeability of the overall coating, and it is also a function of
the materials used. Multilayered coatings with three, four, five or
more individual layers can be used.
The first layer of the multilayered coating 25 is a barrier layer
26. This first barrier layer 26 is followed for example by an
intermediate polymer layer 27. The intermediate polymer layer 27
has the functions of stabilizing the first barrier layer 26 as well
as covering up micro-pores or hairline breaks and thereby reducing
the incidence of these defects in a further barrier layer 26 which
follows the intermediate layer 27. The reduction in the number of
micro-pores or hairline breaks is credited to a certain
surface-smoothing effect of the intermediate polymer layers.
However, the intermediate layer prevents in particular that the
small number of micro-pores or hairline breaks that will still
occur in a second barrier layer 26 attach themselves to the
micro-pores or hairline breaks of the first barrier layer 26, a
condition that would again favor the entry of moisture. Rather,
with the micro-pores or hairline breaks occurring in respectively
different locations in the first barrier layer 26 and further
barrier layers 26, a kind of labyrinth is set up against the entry
of moisture, in particular water or to a lesser extent also solvent
molecules. In a multilayered protective coating 25 with an
alternating sequence of barrier layers 26 and intermediate layers
27, this labyrinth effect leads to a drastic reduction in moisture
penetration.
The barrier layers 26 and intermediate layers 27 of a protective
coating 25 do not necessarily have to follow each other in a
regular sequence, although this can be done because it simplifies
the manufacturing process.
The materials for the barrier layers 26 can be selected from a
multitude of the known, predominantly inorganic insulating
materials that can be applied by different deposition processes.
Examples to be mentioned here are oxides, nitrides, fluorides,
carbides, borides, or combinations thereof, in particular
oxi-nitrides, mixtures of oxides, or also ceramic mixtures. In
particular silicon oxide, titanium oxide, tantalum oxide, zirconium
oxide, hafnium oxide, aluminum oxide, chromium oxide, aluminum
nitride, silicon nitride, titanium nitride, titanium fluoride, and
silicon carbide have proven to be suitable materials for barrier
layers 26. Layers of so-called "diamond-like carbon" can likewise
be used as barrier layers 26.
Other materials that can be used for the barrier layers 26 also
particularly include metals, for example silver, aluminum, gold,
chromium, copper, nickel, titanium, as well as alloys, for example
nickel-cobalt alloys, or intermetallic compounds, for example of
aluminum and copper, of tungsten and copper, or of titanium and
aluminum.
Besides acrylate polymers or inorganic-organic hybrid polymers (as
described for example in EP 0 610 832 A2) there are also further
polymer materials that can be used for the intermediate layers 27.
One could name for example polymeric amides, alkyds, styrols,
xylylenes, phenylenes, aldehydes, esters, urethanes, epoxides,
imides, phenols, ketones as well as fluor polymers or copolymers,
to give only an open-ended listing. In the end, an optimizing
compromise between barrier effect, compatibility of the barrier
layer 26 with the intermediate layer 27, for example their adhesion
to each other, as well as economical aspects of the deposition
method will lead to the decision on which materials to use,
respectively, for the barrier layers 26 and the intermediate layers
27.
Furthermore, one can also use inorganic materials for the
intermediate layers 27 of the multilayered protective coating 25 of
the type shown in FIG. 5. The materials to consider include
predominantly inorganic insulating materials. An inorganic
protective coating 25 that can be mentioned as an example has a
sequence of thin layers of silicon nitride and silicon oxide,
wherein the sequence of 100 to 200 nm of silicon nitride (barrier
layer 26), 100 nm of silicon oxide (intermediate layer 27), 100 nm
of silicon nitride (barrier layer 26), 100 nm of silicon oxide
(intermediate layer 27), and 100 nm of silicon nitride (barrier
layer 26) represents a preferred arrangement, as the silicon
nitride in particular is highly effective as a barrier against the
penetration of moisture. The silicon oxide layer, although this
material can likewise by credited with a barrier property, has the
primary function within the multilayered coating to close off the
possibly occurring micro-pores or hairline breaks in the silicon
nitride layer. Thus, a multilayered coating 25 includes, or
consists of, at least three layers, for example, five layers, of
inorganic materials.
The layers 26, 27 of the multilayered protective coating 25 can
also be composed of an inorganic material including, or consisting
of, at least two components, wherein the stoichiometric composition
of the components changes from one layer to the next. As an example
of materials suitable for multilayered coatings 25 with an
alternating sequence of layers 26, 27 of different stoichiometric
composition, one could name silicon oxi-nitrides with the
respective contents of oxide and nitride varying from one layer to
the next.
A relatively small layer thickness can be selected for purely
inorganic protective coatings 25, as these materials exhibit a
relatively strong barrier effect even for thin layers of silicon
nitride, while on the other hand attention needs to be paid to the
aspect of rigidity, so that the protective coating can adapt itself
to a thermal expansion of the coil if necessary and will not become
defective. Observing the latter constraint, purely inorganic
protective coatings 25 can be applied with an overall thickness of
a few hundred nanometers up to a few microns, in the case of
multilayered coatings with layer thicknesses of 50 to 500
nanometers.
A large selection of deposition methods is available for the
application of a multilayered coating 25. Examples for producing
the barrier layers 26 and/or the aforementioned inorganic
intermediate layers 27 that should be named here include vapor
deposition in vacuum, vapor deposition in air, plasma deposition,
microwave plasma deposition, sputtering, sol-gel methods, chemical
vapor deposition (CVD), combustion chemical vapor deposition
(CCVD), plasma enhanced chemical vapor deposition (PECVD), plasma
impulse chemical vapor deposition (PICVD), as well as
electrochemical deposition which is used in particular for the
deposition of metals.
The following deposition techniques are possible for the
application of intermediate polymer layers 27: spray application,
brush application, and immersion coating, in-situ polymerization of
monomers or oligomers that have been deposited by flash
evaporation, as well as electrophoresis, cataphoresis, or
anaphoresis.
In cases where harsh mechanical wear conditions are expected, the
protective covering is provided with a cover coating (not shown)
which shields the protective coating 22, 25 primarily against
extraneous mechanical influences. Polymers with an especially low
absorptivity for moisture, such as polyacrylates, inorganic-organic
hybrid polymers or silicones, are used with particular preference
for this purpose.
An exemplary embodiment of a coating for the protection of a coil
13 in an inductive sensor has a surface-smoothing undercoating 21
of an inorganic-organic hybrid polymer. This surface-smoothing
undercoating 21 can be applied by means of an immersion process in
which a cylindrical coil 13 rests on rollers while a part of the
cylindrical outside surface of the coil is immersed in the liquid
hybrid polymer which is present in the form of a solution. As the
rollers rotate, the coil revolves in the opposite direction so that
the entire circumference area is uniformly covered by the liquid
polymer. After a hardening phase where the undercoating 21 is
exposed to an increased temperature between 80.degree. C. and
130.degree. C. at which the inorganic-organic hybrid polymer is
crosslinked, the coil is placed in a PECVD coating apparatus and
provided with a protective coating of several layers of silicon
nitride in alternation with silicon oxide. The method of
plasma-enhanced chemical vapor deposition (PECVD) has the advantage
that the coil 13 can be set up in a simple holder device in the
coating apparatus, as the plasma surrounds the entire coil surface
and as a result, the coating material is deposited everywhere on
the coil in a surface-conforming manner.
A coil 13 with a protective coating 30 on the outmost windings,
where the protective coating 30 comprises a coating with a
continuous variation of one or more material parameters indicated
by the gray shading over a surface-smoothing undercoating 21, is
shown in FIG. 6 in a strongly magnified sectional view. In this
embodiment the variation of the material parameter occurs
continuously over the entire coating thickness. The variation
represents a gradient of the chemical composition of the material.
The protective coating 30 was produced in a PECVD coating
apparatus, where the coating material was deposited from two
sources with varying deposition rates, one with silicon oxide and
one with silicon nitride. The rate from one source increased as a
function of time while the rate from the other source
decreased.
The situation is different if the coating material for the
protective coating 22 is, e.g., vapor-deposited or applied by
sputtering, in which case no uniform protective coatings can grow
because of the shadow effect. This problem can be solved by, for
example, revolving the coil 13 in the coating apparatus, so that
all of the surfaces to be coated can be turned towards the material
source.
The coil 13 or coil arrangement 23 that is provided with one of the
coverings of the foregoing description with a surface-smoothing
first level of coverage 21 on which a second level of coverage 22
is applied as a protective coating against moisture penetration has
been described and illustrated in preferred embodiments. However,
based on the teachings disclosed herein, those skilled in the
pertinent art will be able to realize further embodiments. To name
a particular example, the force-measuring cell 1 which has been
described in the context of FIG. 1 and which is provided with a
coated coil does not necessarily have to be equipped with a
force-reducing lever system, as the inductive sensor can also be
connected directly to the vertically displaceable portion 3.
It will be appreciated by those skilled in the art that the present
invention can be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The presently
disclosed embodiments are therefore considered in all respects to
be illustrative and not restricted. The scope of the invention is
indicated by the appended claims rather than the foregoing
description and all changes that come within the meaning and range
and equivalence thereof are intended to be embraced therein.
* * * * *